Sphingosine 1-phosphate (S1P) signals via its cell-surface G protein-coupled receptors to regulate vascular permeability, inflammation, angiogenesis and vascular maturation. In the past funding period, we found that apolipoprotein M (ApoM) is required for S1P to be associated with HDL and that ApoM+HDL activates the endothelial S1P1 receptor to induce vascular homeostasis and inhibit inflammatory responses. Recent findings also revealed a novel function of ApoM+HDL in the suppression of lymphopoiesis. In addition, we uncovered that the S1P1 receptor is a critical proximal component of shear stress sensing in the vascular endothelium and regulates vascular development and homeostasis. This renewal application aims to further elucidate this fundamental signaling system vascular and immune systems. Since S1P receptor modulators are now used in the clinic to treat autoimmune conditions, it is important to fully define this signaling system and to understand the cardiovascular implications. The overarching hypothesis of this proposal is that an HDL-bound ApoM/ S1P complex in plasma activates S1P receptors to regulate physiologic vascular development and homeostasis as well as immune homeostasis. The balanced activation of multiple S1P receptors by HDL/S1P plays a critical role in normal health of the vascular system and if dysregulated, leads to vascular disease. We propose to elucidate the mechanisms by which ApoM+HDL delivers S1P to its receptors on endothelial cells and regulates biological effects. Second, the physiological importance of ApoM+HDL to regulate vascular inflammation via S1P receptors will be examined in mouse and fish models. Biochemical analysis in cultured endothelial cells, genetic loss-of-function studies in zebrafish and genetic mouse models of receptor function will be conducted to further define the key S1P pathway components. Third, we will explore the novel finding that ApoM+HDL signaling via S1P receptors restrain lymphopoiesis. The concept that ApoM+HDL/S1P regulates immune ontogeny while albumin/S1P mediates immune cell trafficking will be tested rigorously. Therapeutic opportunities with reconstituted ApoM+HDL will be explored. Since S1P receptor modulators have entered the therapeutic era, the findings from this project are likely to have rapid translational potential.